The present invention relates generally to the investment casting art, and more specifically to a new investment material for use in making refractory molds.
Investment casting molds are typically made by preparing a pattern assembly or tree comprising of a plurality of patterns having the configurations of the desired metal castings, the patterns being made of wax, plastic or other expendable material. The combination of the tree or runner system and patterns is referred to as the set-up. To form the mold, the pattern or patterns are placed into a flask which is then filled with a refractory investment slurry. The slurry is allowed to harden in the flask around the pattern to form the mold. After the investment slurry has hardened, the patterns are melted out of the mold by heating in an oven, furnace or autoclave. The mold is then fired to an elevated temperature, e.g., 1350.degree. F., to remove water and burn off any residual pattern material in the casting cavities. In many instances, the mold is first cooled to a lower temperature in order to obtain optimum casting conditions before filling it with molten metal. For example, when casting aluminum it is the practice to cool the mold to anywhere from room temperature to about 400.degree. or 500.degree. F. before filling it with the molten metal.
Conventional investment formulations used for non-ferrous molds are comprised of a binder and a refractory made up of a blend of fine and coarse particles. The refractory usually is wholly or at least in part silica, such as quartz, cristobalite or trydymite. Other refractories such as calcined mullite and pyrophyllite can also be used as part of the refractory. The binder is typically fine gypsum powder (calcium sulfate hemihydrate).
The binder and refractory, together with minor chemical additives to control setting or hardening characteristics, are dry blended to produce the dry investment powder. The dry investment is then prepared for use by mixing it with a sufficient amount of liquid, such as water, to form a slurry which can be poured into the flask around the set-up. Vacuuming of the slurry and vibration of the flask are frequently employed steps to eliminate air bubbles and facilitate filling of the flask.
A serious problem encountered with conventional investment molds is the frequent occurrence of cracking during the heating and/or cooling cycles and during the metal casting operation itself. If a vacuum is applied to the molds during pouring of the molten metal, the molds are subjected to additional stresses which can contribute to the cracking. Mold cracking results in metal flash on the castings which must be removed by expensive finishing operations, and permits particles or flakes of investment to break loose and fall into the mold cavities. This can produce inclusions in the castings and cause them to be rejected. In instances where cracking is especially severe, the molten metal can leak through the mold wall so that the entire mold must be scrapped.
Another problem associated with the use of gypsum bonded refractory investments is the tendency for the refractory particles to settle away from underneath the surfaces of the disposable pattern materials after the investing operation is completed, but before the investment has hardened. This type of defect is a source of rejected castings and is believed to be the result of a watery layer that forms between the pattern surface and the slurry like investment when it is still relatively fluid.
Still another source of rejects is concerned with castings that are out of tolerance from a dimensional standpoint. There are many variables involved with dimensional problems including those resulting from sudden volumetric changes that occur due to dehydration of the binder during the firing of the mold, the thermal expansion characteristics of the refractory, and the temperature differential between different sections of the mold. All of these can contribute to the distortion of the mold cavity after the pattern has been eliminated.